The present invention relates to a temperature measurement apparatus in which the temperature of a medium such as air is measured by detecting a change with temperature in its acoustic wave propagation speed.
Heretofore, there have been disclosed such devices according to prior art, in which, an output signal of a carrier wave generator is frequency-modulated by means of a low frequency variable-frequency generator to provide a modulated wave for exciting an ultrasonic vibrator, thus causing an ultrasonic wave of about 40 KHz as shown in a waveform of FIG. 9A to be sent therefrom intermittently; the ultrasonic wave is received by a receiver and amplified to obtain a signal as shown in FIG. 9B. The amplified signal is passed through a frequency discriminator to obtain a modulating wave. The modulating wave thus obtained is compared with respect to its phase so as for its modulation frequency to be varied so that the distance between the acoustic transmitter and the receiver is an integral multiple of the wavelength of the modulating wave and so that the established frequency indicates the propagation speed and thus the temperature of the medium between the transmitter and the receiver.
However, in these prior art devices, a time duration for building up a signal as shown at B - 1 in FIG. 9B, is necessary between the time when the receiver receives the carrier wave and that when an output waveform having a fixed level is established. As a result, when not only the medium to be measured but also the whole measurement circuit are subject to a considerable change of temperature, for example, a waveform shown as C - 1 in FIG. 9C which is received and amplified in the receiver at a temperature of To will be evidently shifted as shown as the waveform C - 2 in FIG. 9C as the temperature is varied to increase the amplification factor of the amplifier. Thus, a phase difference or time difference t is caused due to a change in the performance of the measurement circuit, that is, a problem that a true change in the acoustic wave propagation speed cannot be detected precisely is raised.